Method and plant for drying and comminution of moist, mineral, raw materials

ABSTRACT

Provided is a method for drying and comminution of moist, mineral raw materials, including at least one of chalk, marl or clay. The moist, mineral raw material is dried and comminuted in a drier crusher while simultaneously supplying hot gases from a kiln system to form a dried material. The dried material in suspended form is directed to a first separator in which the dried material is separated into a coarse fraction and into a fine fraction. The fine fraction is directed to a separation cyclone and then to the kiln system. The coarse fraction is directed to a dosage means. The dosage means directs a first portion of the coarse material to the drier crusher and a second portion of the coarse material to a separate grinding unit where any hard material components, including at least one of flint, sand or marble, are subjected to grinding to form a ground material. At least a portion of the ground material from the separate grinding unit is supplied to the drier crusher. Also provided is an apparatus for conducting the method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage entry under 35 U.S.C. 371 ofinternational application No. PCT/IB2006/055046, filed 28 Dec. 2006,designating the United States. This application claims foreign priorityunder 35 U.S.C. 119 and 365 to Denmark patent application No.PA200600279, filed 28 Feb. 2006.

FIELD OF THE INVENTION

The present invention relates to a method for drying and comminution ofmoist, mineral raw materials, such as chalk, marl and clay, by whichmethod the raw materials are dried and comminuted subject tosimultaneous supply of hot gases in a drier crusher from which thematerial is subsequently directed in suspended form to a separator inwhich it is separated into a coarse fraction which is returned to thedrier crusher for further drying and comminution and into a finefraction which is directed to the next stage of the process and whereany hard material components such as flint, sand and marble aresubjected to grinding in a separate grinding unit. The invention alsorelates to a plant for carrying out the method.

BACKGROUND OF THE INVENTION

A method as well as a plant of the aforementioned kind is well-knowni.a. within the cement manufacturing industry. In actual fact, themethod described is used in many cement plants around the world wherethe available cement raw materials mainly consist of soft and wetmaterials comprising chalk, marl and clay which do not necessarilyrequire grinding or comminution into smaller particles but being capableof being reduced to the desired fineness in a wash mill without anymajor complications. In kiln systems of the semi-dry type comprising adrier crusher only a portion of the raw materials is prepared intoslurry prior to drying while the material components which are alreadydry or almost dry are fed directly to the drier crusher. Hence the fuelconsumption will be significantly reduced in comparison with situationswhere all raw materials are prepared into slurry.

These soft raw materials often contain a quantity of hard components inthe form of flint, sand, marble and the like. These hard components maytake the form of relatively large balls of a size equal to a hen's eggand incapable of being crushed to the desired fineness of less than 100μm in the drier crusher. Typically, the drier crusher will only be ableto break down these hard components to a particle size within the rangeof 1 to 2 mm. The raw materials may also contain strips of hard,compressed limestone which may also be difficult to comminute in thedrier crusher. In cases where the raw materials prior to the dryingprocess in the drier crusher are subjected to comminution in a washmill, the hard material components may relatively easy be screened outsubsequent grinding in a separate grinding unit or for subsequentdisposal. The drawback of this method is of course the attendant needfor all raw materials to be prepared into slurry in substantial amountsof water which must subsequently be removed, for example by filtrationor evaporation.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a method as well asa plant which will make it possible to use the drier crusher for thedrying and comminution of soft and wet raw materials containing aquantity of hard material components without any prior preparation ofslurry with addition of extra water.

This is obtained according to the invention by a method of the kindmentioned in the introduction and being characterized in that a partquantity of the coarse fraction from the separator is proportionatelyfed to the separate grinding unit for the grinding of hard materialcomponents.

Hence, it will be possible to substantially reduce the specific energyconsumption and the CO₂ emission from the kiln system due to theelimination of need prior to drying and comminution in the drier crusherto suspend the raw materials in water which must subsequently beremoved. This is ascribable to the fact that the hard materialcomponents by the method according to this invention will be drainedaway from the circuit between the drier crusher and the separator. Afurther advantage of the method according to this invention is thesignificant reduction achieved in the wear rate of the drier crusher andits energy consumption. Furthermore, it will be possible to produce araw meal with excellent burning characteristics due to the reduction inthe content of coarse silica particles in the raw meal, which in turnwill reduce the NO_(x) emission from the kiln.

The material which has been ground in the separate grinding unit can bemixed with the fine fraction from the separator, for example in asubsequent homogenizing silo; however, it is preferred that the materialis mixed with the material suspension which is directed from the driercrusher to the separator. This will increase the probability of the hardraw materials having the desired degree of fineness and dryness.

The hot gases which are fed to the drier crusher may in principle beconstituted by any gases deemed to be appropriate for the purpose.However, in connection with the cement manufacturing process it ispreferred that the hot gases be constituted by hot exhaust gases from asubsequent kiln system, possible mixed with hot air extracted from thecooler of the kiln system.

The plant for carrying out the method according to the inventioncomprises a drier crusher for the drying and comminution of moist,mineral raw materials such as chalk, marl and clay, a source for supplyof hot gases to the drier crusher, a duct for transporting the materialsuspension from the drier crusher to a separator for separating thematerial suspension into a coarse fraction and a fine fraction, a ductfor returning the coarse fraction to the drier crusher for furtherdrying and comminution, a duct for diverting the fine fraction and aseparate grinding unit for grinding any hard material components such asflint, sand and marble, and being characterized in that it comprises adosage means for proportionately feeding a part quantity of the coarsefraction from the separator to the separate grinding unit.

The separate grinding unit may be constituted by any means deemedsuitable for grinding hard material components such as those mentionedabove, and it may, for example, take the form of a ball mill, rollermill or a roller press. The grinding process may be wet or dry.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a plant according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be explained in further details with reference tothe drawing, being diagrammatical, its only FIGURE showing a plant forcarrying out the method according to the invention.

The plant shown comprises two homogenizing stores 1 and 2 for moistlimestone, chalk, marl, clay and similar materials excavated from thequarry, dosage means 3 and 4 for feeding raw materials from the stores 1and 2 to a belt conveyor 5 which transports the raw materials to a driercrusher installation 6. The plant comprises an analyzer 7 for analyzingthe composition of the raw materials and for correcting the dosage means3 and 4 to allow prior determination of the correct chemistry of the rawmaterials. The shown drier crusher installation 6 comprises a driercrusher 8 which receives hot exhaust gases from a subsequent kiln system20 via a duct 9, and a discharge duct 10 which in suspended formconducts the crushed material from the drier crusher 8 to a separator11. In the separator 11 the material is separated into a coarse fractionwhich via a duct 12 is returned to the duct 9 and hence the driercrusher for further drying and comminution, and a fine fraction whichvia a duct 13 is directed to a separation cyclone 14 and an intermediatebin or a homogenizing silo 15. The plant also comprises a separategrinding unit 16, here shown by way of a ball mill, for grinding hardmaterial components, such as flint, sand and marble which are directedto the grinding unit via a duct 17. The material from the grinding unit16 is routed via a duct 18 to the discharge duct 10 in which it is mixedwith the material from the drier crusher and directed to the separator11. Alternatively, the material from the grinding unit 16 may bediverted in relation to the separator 11 allowing it to be introducedinstead into the duct 13 if it can be definitely ascertained that it hasthe desired degree of fineness and dryness.

The shown kiln system 20 which is not a part of the present inventionconsists of a traditional kiln system comprising a cyclone preheater 21,a calciner 22, a rotary kiln 23 and a clinker cooler 24 in which the rawmaterials from the intermediate store 15 in known manner are preheated,calcined, burned into cement clinker and cooled. The gas transportthrough the rotary kiln 23, the calciner 22, cyclone preheater 21, thedrier crusher 8, the separator 11 and the separation cyclone 14 iseffected by means of an exhaust gas fan 28.

By way of illustration, the shown plant also comprises a dosage means 25for introducing dry ingredients such as fly ash and iron oxide into thedrier crusher installation 6. The shown plant also comprises a dosagemeans 27 for hard material components occurring in reasonably pure form,i.e. not encapsulated in the soft raw material components which areintroduced via the dosage means 3 and 4. The hard material componentsare directed to the grinding unit via the duct 17.

According to the invention a part quantity of the coarse fraction whichvia the duct 12 leaves the separator 11 is proportionately fed by meansof a dosage means 26 via a duct 19 to the separate grinding unit 16 inwhich it is ground together with any hard material components receivedfrom the dosage means 27.

The draining away of the hard material components to prevent them frombeing returned to the drier crusher 6 will significantly reduce the wearrate of the crusher and its energy consumption. Stable operation of thegrinding unit 16 will be ensured by proportionately feeding a partquantity of the coarse fraction from the separator 11.

Another advantage is the elimination of the need to prepare the rawmaterials into slurry in order to separate hard material componentsprior to the drying and comminution in the drier crusher which willreduce the necessary energy consumption in connection with thesubsequent removal of the water, hence reducing also the CO₂ emissionfrom the kiln system. Also, it will be possible to produce a raw mealwith excellent burning characteristics due to the reduced content ofcoarse silica particles in the raw meal, which in turn reduces theNO_(x) emission from the kiln.

The invention claimed is:
 1. A method for drying and comminution ofmoist, mineral raw materials comprising: drying and comminuting a moist,mineral raw material in a drier crusher while simultaneously supplyinghot gases to form a dried material; directing the dried material insuspended form to a separator in which the dried material is separatedinto a coarse fraction and into a fine fraction; directing the finefraction to a next stage; directing the coarse fraction to a dosagemeans, the dosage means directing a first portion of the coarse materialto the drier crusher and a second portion of the coarse material to aseparate grinding unit where any hard material components are subjectedto grinding to form a ground material; and supplying at least a portionof the ground material from the separate grinding unit to the driercrusher, wherein the hot gases comprises hot exhaust gases from asubsequent kiln system.
 2. The method according to claim 1, wherein themoist, mineral raw material comprises at least one of chalk, marl orclay.
 3. The method according to claim 1, wherein the hard materialcomponents comprises at least one of flint, sand or marble.
 4. Themethod according to claim 1, wherein the material which has been groundin the separate grinding unit is mixed with the material suspensionwhich is directed from the drier crusher to the separator.